Cantor-Lebesgue theorem for double trignometric series

Let ∥·∥ be a norm in R² and let γ be the unit sphere induced by this norm. We call a segment joining points x,y ε R² rational if (x₁ ? y₁)/(x₂ ? y₂) or (x₂ ? y₂)/(x₁ ? y₁) is a rational number. Let γ be a convex curve containing no rational segments. Satisfaction of the condition $$T_\nu (x) = \sum\nolimits_{\parallel n\parallel = \nu } {c_n e^{2\pi i(n_1 x_1 + n_2 x_2 )} } \to 0(\nu \to \infty )$$ in measure on the set e? [- 1/2,1/2)×[- 1/2, 1/2) =T² of positive planar measure implies ∥T _v ∥L₄ (T²) → 0(v → ∞). if, however, γ contains a rational segment, then there exist a sequence of polynomials {T _v } and a set E ? T², ¦E¦ > 0, such that T _v (x) → 0(v → ∞) on E; however, ¦c_n¦ ? 0 for ∥n∥ → ∞.     

On Voiculescu's double commutant theorem

For a separable infinite-dimensional Hilbert space , we consider the full algebra of bounded linear transformations and the unique non-trivial norm-closed two-sided ideal of compact operators . We also consider the quotient -algebra with quotient map

For any -subalgebra of , the relative commutant is given by for all in . It was shown by D. Voiculescu that, for any separable unital -subalgebra of ,

In this note, we exhibit a non-separable unital -subalgebra of for which (VDCT) fails.

     

Hilbert重级数定理的一个新推广

通过引入一个适当的对数函数建立了一种新的Hilbert型不等式.利用Euler-Maclaurin求和公式对权函数进行了估计.证明了常数因子π2r＋1Er是最佳的,其中Er是Euler数.作为应用,给出了一些互相等价的不等式.     

On certain polylogarithmic double series

In this paper, we study certain polylogarithmic double series

Hilbert's double series theorem extended to the case of non-homogeneous kernels

Sharp version of celebrated Hilbert's double series theorem is given in the case of non-homogeneous kernel. The main mathematical tools are: the integral representation of Mathieu's (a,λ)-series, the Hölder inequality and an extension of the double series theorem by Yang.     

On Bohr's theorem for multiple Fourier series

The problem of Pringsheim uniform convergence of multiple Fourier series in the trigonometric system is considered. A multidimensional analog of Bohr's theorem on the uniform convergence of the Fourier series of a continuous function after a homeomorphic chance of variable is proved.Translated fromMatematicheskie Zametki, Vol. 64, No. 6, pp. 913–924, December, 1998.     

On general summability factor theorem of infinite series

A lower triangular matrix with nonzero principal diagonal entries is called a triangle. In this paper we obtain the sufficient conditions for ∑a_nλ_n to be summable ∣A∣_k whenever ∑a_n is summable ∣T∣_k for a triangle T.     

On uniformly convergent double Fourier series

The uniform convergence of the partial sums, relative to a system of homothetic starlike polygons, is considered. Among other things, it is established an arbitrary preassigned positive sequence from can be majorized by the coefficients of a uniformly convergent double Fourier series.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova Akademii Nauk SSSR, Vol. 178, pp. 151–162, 1989.     

On the convergence of double orthogonal series

={_i()} ={_i(y)} (i=1,2,...) — (, ) . (,)Х(,) {_i(x)_k(y))} (i, k=1, 2, ...). (1.1) , . , , . , , . (i) (1.3), .. « » (1.1), (1.7) . , (ii) « » (1.4) (1.1), (1.8) .     

On alternating analogues of Tornheim's double series

In this paper, we give some evaluation formulas for alternating analogues of Tornheim's double series. These can be regarded as alternating analogues of Mordell's formulas. This gives a partial answer to the problem posed by Subbarao-Sitaramachandrarao.

     

On the Fabry ratio theorem for orthogonal series

It is proved that the Fabry ratio theorem for power series is valid under quite general assumptions and for orthogonal series.     

On the harmonic summability of double Fourier series

Summary The author studies a problem relating Harmonic Summability of doubleFourier serie analogous to theGergen's criteria for convergence.     

On the Lebesgue summability of double trigonometric series

We recall that the Lebesgue summability of a single trigonometric series is defined in terms of the symmetric differentiability of the sum of the formally integrated trigonometric series in question. In this paper, we present another proof of the theorem given in Zygmund's monograph. Then we define the notion of Lebesgue summability of a double trigonometric series and extend the theorem of Fatou and Zygmund from single to double trigonometric series.